Rigify Generate -- Deformation Bones vs. Armature?
When you generate the rigify rig after adjusting the initial meta-rig and pressing generate in the armature tab (in the properties window), rigify genetates a host of control bones with ready made complex behaviors such as IK, foot roll, fist open/close, and many more.

If you're animating the deform bones directly, and ignoring all these features, you probably don't need to generate and can bind to the meta-rig directly.